In recent years industries such as aircraft, automotive and electronics have created a demand for very high precision metal parts. It is becoming increasingly common to specify dimensions in thousandths-of-an-inch rather than hundredths-of-an-inch as was the general practice a decade ago. Drilling and boring of precision holes in metal parts in particular requires accurate measurements to ensure that the inside hole diameter tolerance is satisfied for all depths, that the out-of-round tolerance is not exceeded and that the surface roughness is within specification.
The best commercially available bore diameter measuring instruments are hand operated, mechanical contact devices with digital readout, and the accuracy of the measurements they produce usually depends upon the skill of the operator. Using these instruments out-of-round measurements are achieved through multiple readings along different diameters and the accuracy of the measurements tends to be subjective.
Very high precision measurements of bore holes and the like can be made using coherent optical interferometric techniques employing lasers as the coherent light source, but these instruments are delicate, expensive, slow and not generally amenable to hand-held use. A current paper, Dresel et al, "Three-dimensional sensing of rough surfaces by coherence radar", Applied Optics, 31, 919-925 (1992), not known to be prior art discloses optical interferometric techniques not limited to coherent light and teaches acquiring three dimensional data of rough objects as well as the use of wide band radiation.